Off-equatorial deep cycle turbulence forced by Tropical Instability Waves in the equatorial Pacific cold tongue

Abstract:

Being cold surface water in a region of large solar insolation, the equatorial Pacific cold tongue is a region of large heat uptake by the ocean (e.g. Holmes et al., 2019). Such heat uptake is only possible if the heat absorbed at the surface by the ocean is quickly exported away. Microscale turbulence contributes to this export by transporting heat downward and is thus essential for keeping the cold tongue (e.g. Moum et al., 2013; Wang and McPhaden, 1999).

Observations at 0°N, 140°W repeatedly show the presence of a daily cycle of shear-driven turbulence — “deep-cycle turbulence” — in the strongly sheared marginally stable flow (gradient Richardson number Ri ≈ 0.25) above the core of the Equatorial Undercurrent (EUC) but beneath the base of the surface mixed layer. Deep-cycle turbulence was also found to be modulated by Tropical Instability Waves (TIWs; Moum et al, 2009).However these microstructure observations have only been recorded at a few specific locations on the equator.

Here we report the presence of an off-equatorial deep-cycle turbulence forced by TIWs in a 1/20° realistically-forced regional MITgcm model. TIWs enhance the meridional component of vertical shear (∂v/∂z) resulting in deep cycle turbulence in the eastward extension of TIW cold cusps (usually between 3°N and 5°N). The enhanced ∂v/∂z is a result of the rotation (tilting) of ∂u/∂z (horizontal vorticity) initially generated by vortex stretching of SEC-EUC ∂u/∂z near the equator. Indirect observational evidence for this TIW-forced off-equatorial deep cycle is seen in Richardson number profiles from historical cruise sections crossing TIWs at 110°W which show Ri ≈ 0.25 at 4°N or 5°N. The off-equatorial marginal stable layer is visible throughout the longitudinal extent of the simulated cold tongue, and expands the region of oceanic heat uptake to outside the zone of direct EUC influence (approximately 3°S - 3°N).

These results emphasize the need to observe turbulence variability off the equator. Ongoing efforts are targeted at studying the off-equatorial deep cycle using large eddy simulations; and constraining its importance to the cold tongue's heat budget.